On Tuesday, the leaders of the nonprofit L.A. River Revitalization Corp. used a riverside press conference at North Atwater Park to trumpet its plan to complete a continuous bike path and greenway along all 51 miles of the river, extending from the San Fernando Valley to Long Beach, by 2020.

The group also said it has raised more than $5 million to build a pedestrian, bike and equestrian bridge that would span the river between Atwater Village and Griffith Park.

Yet the scope of those projects is minor compared to proposals being prepared by the U.S. Army Corps of Engineers, which holds ultimate decision-making power over the river. The Corps is putting the finishing touches on a much-anticipated, much-delayed feasibility study focusing on an 11-mile stretch of the river between Union Station and Griffith Park.

The study, expected to be released Aug. 30, will weigh three ambitious plans aimed at restoring the river's natural ecosystem and improving public access to its banks.

The plans could fundamentally change how the public looks at the river, which has been more of an eyesore than natural amenity since the federal government wrapped much of it in concrete as a flood-control measure beginning in the 1930s.

Crucially, the river is also the last great source of new green space in Los Angeles, a city notoriously short on parks. One benefit of the greenway plan is that it would bring thousands of cyclists and pedestrians up to the edge of the river, building a constituency for further improvements.

The cheapest of three Army Corps options under consideration, at $444 million, so-called Alternative 13 calls for concrete removal and ecological restoration at several key sites along the 11-mile stretch, including the Taylor Yard complex on the east side of the river, near Glassell Park.

Alternative 16, with an estimated price tag of $774 million, would widen the river to accommodate terracing on much of its eastern bank (from roughly Los Feliz Boulevard downstream to Fletcher Drive) and redevelop the so-called Piggyback Yard, a site that is owned by Union Pacific Railroad and covers 125 acres across the river from Union Station.

Alternative 20, at $1.06 billion, would, in addition to those changes, provide new connections to the L.A. State Historic Park near Chinatown, known as the Cornfield, and fund substantial ecosystem restoration where the river meets a tributary, the Verdugo Wash, near the border of L.A. and Glendale.

The percentage of each project that Washington would pay for varies from plan to plan. For the cheapest, Alternative 13, the city would have to bear more than 70% of the total cost, including land value. Each of the other options would come with more federal money, closer to a 50-50 split.

Corps officials would not agree to speak on the record about the upcoming study. "We cannot comment on details of the report prior to public release," Jay Field, a spokesman for the L.A. District office of the Corps, said in a statement.

But others who have worked on plans for the river say the Corps is likely to endorse the cheapest and least ambitious of the three options. Doing so would disappoint leaders of the L.A. office, who not only pushed hard for Alternative 20 but expected it to win.

"I don't think the local office anticipated the resistance they got up the chain" to the more expensive alternatives, said Carol Armstrong, director of the Los Angeles River Project Office for the city's Bureau of Engineering.

She pointed out that the three options being considered by the Army Corps have emerged from a larger pool of alternatives, with some carrying price tags as high as $7 billion.

The feasibility study has been underway since 2006, and many in Los Angeles have grown weary of waiting for it to be completed. But if the study is released in August as expected, the timing may turn out to be surprisingly good, building on recent progress like the greenway plan.

Politicians and advocates for the river are likely to argue that federal investment will complement major infrastructure and design projects planned in or near downtown. These include a new master plan for the area around Union Station; the potential arrival of a high-speed rail; a $400-million replacement for the 6th Street Bridge, expected to open in 2019 with substantial new park space along and near the river; and the expansion of bike paths around the city.

Plans for the river are also in line with environmental and infrastructure programs promoted by the Obama administration, including its Great Outdoors initiative, an Interior Department project aimed at supporting grassroots conservation and recreation activities.

There is even increasing talk at City Hall and elsewhere that a bid from L.A. to host the 2024 Summer Olympics could include new facilities near or along the river. The U.S. Olympic Committee is expected to name a shortlist of contenders for the 2024 Games by the end of this year.

Angelenos' attitudes about the river appear to be growing more positive, even protective. When a tanker truck crashed and caught fire on July 13, sending burning gasoline through a storm drain and into the river, the reaction on Twitter suggested that many people were more concerned about the accident's effect on the waterway than how long freeways might be closed. That sense of affection was practically nonexistent a decade ago.

The Environmental Protection Agency in 2010 officially declared the river a "navigable waterway," and it is now possible to take kayak trips along a short stretch, as Los Angeles Mayor Eric Garcetti made a point of doing during his first week in office.

Still, that momentum may founder on the same bureaucratic obstacles and red tape that have slowed improvements to the river for decades.

Two nonprofit organizations are dedicated to improving it: Friends of the Los Angeles River, founded in 1986, and the L.A. River Revitalization Corp., created by the city of Los Angeles in part to pursue public-private partnerships. The two groups don't always see eye to eye.

There are also two official master plans for the river: one produced by Los Angeles County in 1996, another approved by the city in 2007.

The battle won't end when the Corps releases the feasibility study next month. A 45-day period of public comment will follow, giving river advocates a chance to argue for the more expansive vision of Alternative 20, if that is not selected.

The mayor has already been making the case in private.

"I've been talking to Sen. Boxer and the White House and emphasizing the importance of this study," Garcetti said. "I told them that this is a real grassroots effort that has culminated in this moment, and that we would hope that we wouldn't just get the cheapest version."

There is a sense among longtime river-watchers that the potential for transformative change — and federal help in that effort — has never been greater. But optimism is accompanied by worries that Washington won't quite rise to meet the moment.

And bureaucratic and jurisdictional knots will take time to fully unravel. Even at the Tuesday press conference, accompanied by the gritty soundtrack of traffic noise from nearby Interstate 5, this was plainly evident.

For all the benefits the pedestrian bridge will bring, it won't connect directly to Griffith Park. Getting from Atwater Village to the park will require first crossing the new bridge and then using a tunnel under the freeway.

A single span crossing both river and I-5, apparently, was something of a bridge too far.